I relocated to Boise from the East Coast a few years ago and visited SLC and Boise each five times before I made my decision on Boise. Here is a little insight.
If you are a non driver then the public transport system in SLC will suit your needs better than the system in Boise, which is getting better but still needs some work.
Each city has great outdoor activities--SLC has close access to skiing in the winter, but Boise is adjacent and much closer to vast mountainess and forested alpine scenery while SLC is more desert and dry. The Wasatch mountains are pretty, but when you fly over them there is desert valley on one side and desert valley on the other side, they are a narrow mountain range. The mountains above, adjacent, and near Boise are far more beautiful than the Wasatch, there are a lot more mountains near Boise than there are near SLC. The mountains above and behind Boise roll on forever, actually all of the way north up the length of the state and into Canada, as well as the verdant forests, and there are a lot more rivers and natural lakes than you will find in Utah. All of this nature is one of Boise's greatest treasures. But both cities and areas are pretty, everyone has their own opinion. There is skiing above Boise on the top of the mountains and there are beautiful resorts in the mountainess areas outside of Boise, skiing in Idaho is a true alpine experience with tons of pine trees to maneuver through. If you like to fish you are in luck, this is Idaho after all, you can fish for Steelhead trout and salmon in the Boise River right in the city in view of downtown highrises. Boise is a city full of outdoor activities, huge lush city parks, people jogging and riding bikes; bikes are everywhere and you almost have to own a mountain bike to fit in with the rest of Boise... and scooters are a popular means for transportation here too.
Boise has a large concentration of large national/international companies/headquarters and is also full of small and midsize businesses. Boise is often ranked as one of the best entreprenuerial cities, and so is SLC. Both are midzised cities, SLC has a larger metro area, but Boise has a more urban and often described as a more sophisticated downtown, not as many skyscrapers as SLC, but real life takes place at street level. Boise is very pedestrian friendly and boutiquish with many, many business and restaurants in storefronts all over downtown, not just confined to some malls like you find in SLC. Both cities have very educated and business minded people. If you like good coffee and a good glass of wine while looking at art then Boise is the place for you, there are nearly two dozen art galleries in the downtown area. Boise is a very creative city with a large population of 20 and 30 somethings. The city boundaries of Boise kind of lack in LDS population while the suburbs have a lot of LDS people. Catholics are the largest religion in Boise.
Both cities are pretty white as far as race, but SLC will have more diversity because the metro population is larger. Boise has a fast growing Asian and African American, and Eastern European community which are locating here from larger cities, and there are a lot of refugees placed here by agencies to start their new lives in America. There are a lot of prominant and very respected African Americans in the Boise community. Boise is a diverse, opened minded fast growing city. People are very friendly and calm.
Since Boise has a smaller metro area with less smokestacks then the SL Valley the air is cleaner, although there are inversions on certain days in the Winter.
Hope this helps even though my reponse is long

Both cities are great places, there are similarities, major differences, and I have noticed people from each city have a lot of pride.
PM if you would like more info.